Why does Blender's tracker keep changing my focal length?
It sounds like you probably have your movie clip set to the default focal length of 24mm:

Blender will of course set your camera to match your movie clip when you run the track, or else the track won't make any sense.
The focal length converges on 35mm after running multiple tracks as Blender eventually figures out that the 24mm setting you've provided it is incorrect.